One of the major advances of the Twentieth Century agriculture has been the improved transportation of produce and other foods from one geographical region to another. Such transportation has provided, for example, fresh fruits and vegetables during winter months by transporting such materials grown in the southern hemisphere to the northern hemisphere and vice versa. However, this increase in transportation of food products has also led to the introduction of insect pests from one area of the world to other areas where the natural predators of that pest do not exist. The economic dangers of importing distant pests into a valuable agricultural region, often through the importation of produce and other food substances, are well known.
The U.S. Department of Agriculture regulates transportation of agricultural products into the United States and from infested to uninfested areas within the United States. In a specific example of such regulation, quarantine regulations require treatment of papaya after harvest for shipment from Hawaii to the U.S. mainland or Japan. Quarantine treatments are used to disinfest the fruit of Mediterranean Fruit Fly, Ceratitis capitata (Wiedemann); Melon Fly, Dacus cucurbitae Coquillett; and Oriental Fruit Fly, D. dorsalis (Hendel).
Prior to 1984, ethylene dibromide (EDB) fumigation was the principal treatment for quarantined fruit. However, the U.S. Environmental Protection Agency discontinued the registration for such use after the harvest in September of 1984 because of concerns relating to carcinogenicity of EDB. EDB was the only quarantine treatment for papaya (and numerous other fruits) in use at that time. The standard treatment consisted of a 20-minute immersion of fruit into 49.degree. C. water for decay control after harvest, followed by an 8 g/m.sup.3 EDB fumigation for 2 hours.
Following the ban on EDB fumigation, two high-temperature treatments were approved for disinfestation of papayas and other fruits. One quarantine treatment is a 2-stage hot-water immersion that provides quarantine security only against fruit fly eggs (Couey and Chew, J. Econ. Entomol. (1986) 79:887-90) This treatment requires harvesting a fruit that is less than 1/4 ripe, before the papayas become susceptible to fruit fly infestation. Harvest at this ripeness prevents potential larval infestations that cannot be controlled by the 2-stage hot-water immersion, but the fruit is less than optional at this stage of ripeness.
The other quarantine treatment is a "quick run-up" vapor heat treatment first described by Baylock and Kozuma, "Special Report No. 7, Fruit Fly Investigations in Hawaii," U.S. Department of Agriculture, Entomology Research Branch, Honolulu, Hawaii (1954). This process consists of a short preheating (preconditioning) period using hot air at 40-60 percent relative humidity to a specified temperature. The fruit pulp is then gradually warmed to 47.2.degree. C. (117.degree. F.) using high-temperature air saturated with water vapor. The "quick run-up" treatment does not require fruit selection at an early stage because it is effective against both the egg and larval stages of C. capitata, D. cucurbitae, and D. dorsalis. Unfortunately, both the 2-stage hot-water immersion and the "quick run-up" vapor heat treatments can cause damage to papayas and other fruits, even under precisely controlled conditions.
More recently, proposals have been set forth for steam treatment of fruit to kill insects. One such apparatus is described in U.S. Pat. No. 4,716,676, in which steam is injected into the apparatus and a humidity of 90-100 percent is maintained during the treatment process. However, this apparatus is expensive and further requires exposing produce to simultaneous heat and humidity much in the same manner as the vapor heat treatment described above.
In response to these problems of fruit damage, the U.S. Department of Agriculture has developed a high-temperature, forced-air (HTFA) quarantine treatment for papayas infested with various fruit flies and other insects. See, for example, PPQ Treatment Manual, USDA, especially Sec. III, "Treatment Procedures," Part 15, "High Temperature Forced Air," and Sec. IV, "Treatment Facilities," Part 6, "High Temperature Forced Air," (version of March, 1990); see also Gaffney and Armstrong, "High Temperature Forced-Air Research Facility for Heating Fruits for Insect Quarantine Treatments," J. Economic Entomology (1990) 83: 1959-1964; Armstrong, et al., "High-Temperature Forced Air Quarantine Treatment for Papayas Infested with Tephritid Fruit Flies," J. Economic Entomoloqy (1989) 82: 1667-1674; all of which are herein incorporated by reference. In this process fruit is treated with hot air until the fruit center reaches a temperature of 47.2.degree. C. (117.degree. F.) to 48.8.degree. C. (120.degree. F.) for periods of 2 to 7 hours. In some treatments, heating occurs in set temperature stages, while in other treatments a single temperature sufficient to raise the fruit center temperature to the desired limit is used. Using these treatment conditions with a relative humidity of 40-60 percent, no fruit damage was seen. When the fruit center temperatures reached the desired level, the fruit was immediately hydro-cooled until the fruit centers were 30.degree. C. or below. Phytotoxicity tests demonstrated that the HTFA treatment was not detrimental to fruit quality. Survival tests indicated that the HTFA treatment was sufficient to disinfest fruit such as papayas from C. capitaty, D. cucurbitae, and D. dorsalis infestations.
Although this process has proven useful in providing a treatment of papaya and other easily damaged fruit, disadvantages still exist, particularly with regard to commercial application of the process. The treatment has not previously been amenable to bulk processing of produce. For example, a recent paper by Winkelman and Williamson presented at the 1990 International Summer Meeting sponsored by the American Society of Agricultural Engineers (June 24-27, 1990), which discusses the HTFA process and suggests improvements, indicates that fruit loaded in standard bulk bins showed a relatively large difference in final temperatures in fruit centers for interior vs. exterior fruit in bulk bins. This paper, as have previous papers including the original USDA proposal, recommends shallow, single-layer trays of fruit. However, preparing fruit in single layers is time and labor intensive and does not lend itself to commercial applications. Accordingly, methods and apparatuses for killing insects using the HTFA system as modified to allow for bulk processing are highly desirable.